1. Technical Field
The present disclosure relates to an injection molding process and, in particular, to a gas assisted injection molding process for forming molded articles having a hollow portion formed in a closed geometric portion thereof, for example, a rim of a pot, container or the like. In addition, the present disclosure relates to a molded article and, in particular to a molded article having a hollow portion formed therein as a result of the gas assisted injection molding process.
2. Description of Related Art
Gas injection molding techniques are generally known, such as the technique described in U.S. Pat. No. 5,401,459. Gas injection molding can generally be described as a process where gas is injected into a plastic mold which holds hot molten plastic in order to totally fill the mold cavity, thereby providing a plastic part that is hollow. By way of example, if the typical gas injection molding technique is applied to the creation of a tube, the mold may consist of a cavity which would ultimately result in a solid bar being formed. The hollow cavity of the mold is then completely filled with molten plastic, after the cavity is injected with the molten plastic an inert gas, under high pressure, is injected into the filled mold creating a cavity therein. The excess molten plastic being simultaneously expelled through a gate into a spill cavity. More recently, a “short shot” technique of gas injection molding has been developed. During the “short shot” technique, the mold is first injected with a hot plastic, in an amount less than enough to fill the cavity of the mold. After injection of the “short shot” amount of plastic, the plastic is injected with an inert gas at high pressure. The initial “short shot” of plastic would have just begun to cool slightly at the outer edge such that the gas will blow the center of the plastic along the mold, causing the plastic to fill the mold and simultaneously creating a cavity within the tube being formed.
Typically, the total amount of plastic required for the “short shot” is determined by calculating the total volume of the mold cavity, then, determining the total weight of a solid molded article based on the specific gravity of the plastic to be injected into the mold; determining the volume of gas which is desired to replace the volume of plastic; converting the volume of gas into a percentage of the total volume and reducing the total weight of the plastic which would normally be injected to completely fill the mold by this percentage.
Until recently, the manufacture of articles having a thick hollow ring relative to the surrounding wall thickness have typically needed to be made out of clay, rotational molding or blow molding to name a few. However, the manufacture of these articles has been far beyond the normal limits of a conventional injection molding processes. The application of conventional gas assisted injection molding techniques to the manufacture of such articles results in visible volumetric shrinkage, require significant material consumption and molding cycle time, and produce a great deal of strain on the injection molding machine. Thus, in order to overcome all of these drawbacks, and others, the need exists for a gas assisted injection molding process which more efficiently produces such hollow rimmed articles.